The applications of wire-free communication techniques have actually been increasing almost exponentially in the last two decades. This has now led to both speech services and data services being transmitted in frequency bands including, the 400, 800, 900, 1800 and 1900 MHz bands, which are available worldwide for mobile speech transmission. This frequency range has been extended to 2170 MHz with the introduction of UMTS Standard (Universal Mobile Telecommunication System). As an alternative to landline telephony, the frequency range between 3400 and 3600 MHz, keyword WLL (Wireless Local Loop), has been released in various European countries in previous years. In addition, transmission of high data rates can now be done without the use of wires, using the WLAN frequencies (Wireless Local Area Network). The frequencies released for these applications are in the 2.4 and 5.5 GHz range.
In order to supply all of these services efficiently to an in-house area, such as commercial buildings, airports, railroad stations, underground garages and hotels, an entire forest of antennas would be necessary if individual antennas were to operate exclusively in each relevant frequency band. However, a forest of antennas such as this is highly complex in terms of the space required, installation and operation. Thus, it is desirable to minimize this forest of antennas as far as possible by the use of particularly broadband antennas. One shape which is particularly suitable by virtue of its simplicity is the patch antenna, in which a patch plate, which is arranged above a conductive base surface, is used as an antenna element. In contrast to a monopole antenna, this concentrates the emitted energy in a smaller spatial angle.
Patch antennas have been described in numerous documents and articles (see, for example, the “Microstrip Antenna Design Handbook”, Artech House, Boston London, 2001, pages 8-9 and 16-17). Patch antennas are distinguished by their flat design, which can be produced at low cost. Various basic shapes of patch antennas can also be found in U.S. Pat. No. 6,317,084. One important disadvantage of patch antennas in comparison to other antenna shapes is, however, that their bandwidth is relatively narrow. A patch antenna typically results in bandwidth ratios of 1:1.2 for a VSWR (Voltage Standing Wave Ratio) of <2. Extensive efforts have therefore already been made in the past to widen the bandwidth of patch antennas. Some of the solutions proposed for this purpose are quoted and discussed in the introductory part (columns 1-3) of U.S. Pat. No. 6,317,084, but lead to comparatively complex antenna structures, without being able to comply completely with the broad bandwidth requirements.